My thoughts on this are as follows: Should we be basing any of our discussions on voting population? I've never once voted on the Hugos and never really wanted to (no offense to the writers, I just don't want to pay to be able to vote for a book). How many people are the same way? Also, in reference to the graph, that doesn't really cover anything beyond a basic representation of the facts, and leaves out the fact that there aren't a lot of writers in their twenties or thirties in the first place.
I do believe there is a generation gap, but I think it will be nearly impossible to pin down to an exact percentage or number. There's no way to be entirely certain that old folks only read old folks and young folks only read young folks. Heck, did anyone consider that a lot of readers might not even know what the hell the Hugos even are? Seriously. I didn't know what the Hugo and Nebula awards were until I picked up a copy of Ender's Game and read it on the cover. Even then I just went "oh, it's an award, cool!" It wasn't until my early twenties that I knew what the awards were and started to become more involved in the writing world. A lot of people may not even know the Hugos exist. After all, it's not like the Hugos are on television or on the radio. The only time you hear about them is online or on a book and very rarely in regular conversation with folks.
There are just too many factors in all of this for us to be even remotely capable of pinning down what the generation gap actually is. There probably is a gap, but I don't know if it's a significant one. It might be, though. Granted, I don't know everything and perhaps I'm missing some valuable points on all this. What if the gap is accidental? It's not like writers advertise their age's, unless they happen to be very young and it's needed to boost sales. I read just about everything, regardless of age, but I'm also not the average reader, I suppose.
What do you all think about this? Do you feel there is a big generation gap in SF?